I trained my dogs to ring a bell mounted on the wall near the door.My biggest issue with Fuji was her never asking to go potty, she'd just look at you (when you weren't looking at her) then trot off to pee on the floor because you didn't take her out.
Take them out regularly, with the whole "go pee" verbalizations and happy dancing upon elimination thing, and bop their nose against the bell on the way out.